Investigation On The Preparation And Properties Of Coated ?-Ce₂S₃ Red Pigments

With the enhanced environmental protection consciousness of our human,inorganic pigments with toxcity hevay metal elements has been strictly restricted by environmental laws,red pigment?-Ce₂S₃ is supposed to be a preferred candidate to replace cadmium red pigments for its advantages of nontoxicity,bright red color and resistance to ultraviolet radiation.Presently,?-Ce₂S₃ has been widely used in coating,printing and plastic industry.However,it is unfortunate reported that?-Ce₂S₃ is easy to be oxidized at temperature higher than 350°C,which greatly limits its practical applications.Therefore,to improve the anti-oxidization temperature is the main research content of the?-Ce₂S₃red pigments currently.In view of the current situation and requirement for inorganic pigmengs,this research focused on the preparation of coated?-Ce₂S₃ red pigments,which mainly relates to the follows:Syntheses of SiO₂ coated?-Ce₂S₃ red pigments based on St?ber method,the effect of water/ethanol volume ratio and tetraethoxysilane?TEOS?concentration on the microstructure of the coating layer were investigated.The mechanism for the formation of coating layer was well analysed.The results indicate that a 60 nm uniform dense silica layer can be obtained when the ethanol/water volume ratio and TEOS concentration was100/20 and 25 mg·ml^-1,respectively.Compared with the uncoated pigments,the oxidization resistant temperature of the coated pigments was enhanced about 200 ^oC.A red color?L*=29.59,a*=27.53 and b*=27.66?was still remained for such silica coated?-Ce₂S₃ pigments after heated treated at 550 ^oC in air.In addition,ultra-thick SiO₂ coated?-Ce₂S₃ red pigments were obtained by repeating the coating process.A 140 nm thickness coating layer can be achieved after three times coating.This pigment co-sintered with Al₂O₃-B₂O₃-SiO₂-ZnO low temperature frit at 620 ^oC in air can retained a red color of L*=30.28,a*=32.45 and b*=26.40.However,the uncoated pigment was oxidized to be white completely.It is considered that the molten frit may protect pigments powders by closing the pores in the amorphous silica coating layer.The laser particle size analysis results indicated that the coated pigments possess a more uniform size distribution and better size distribution.Zirconia encapsulated?-Ce₂S₃ pigments were prepared via alkoxide-hydrolysis method by using Zirconium n-butoxide as raw material,and the mechanism for the formation of ZrO₂ coating layer was analysed.The results indicated that the thickness of coating layer increasing with increaing the concentration of Zirconium n-butoxid in the solution.A good coated effect was gained in 100 ml ethanol,when 0.5 ml acetic acid?2M?was used to control the reaction rate,and the accordingly Zirconium n-butoxid concentration was 15ml·L^-1.The DLS analysis result showed that the dispersity of the coated pigments was improved.And then the visible light reflectance curves demonstrate that the reletance in red light region was enhanced from 62.43%to 68.80%,with an accordingly L*,a*and b*enhancement??L*=6.11,?a*=1.53 and?b*=0.84?,which suggested that the color rendering ability was improved by zirconia coated.Unfortunately,there was no obvious improvement in thermal stability of zirconia coated pigments,which should be due to that the coating layer is not dense enough.Based on the above research,the author tried to prepare zircon coated?-Ce₂S₃ red pigments via step control method by using?-Ce₂S₃@SiO₂ and?-Ce₂S₃@ZrO₂ coated pigments as starting red hue.The TEM and SEM analysis indicated that a propective zircon coating layer was not observed when the raw red hue was?-Ce₂S₃@SiO₂,which should be due to the formation of zircon is limited by the Si⁴⁺diffusion to ZrO₂ layer.While the SiO₂ can not even form a stable coating layer on the surface of ZrO₂ layer when?-Ce₂S₃@ZrO₂ was used as the starting red hue,because the ZrO₂ coating layer was not dense enough,.Then,based on the combination zircon formation process and characteristics of?-Ce₂S₃,several coating models were proposed.